A Guatemalan boy in U.S. Customs and Border Protection custody died Monday, the agency announced in a statement.

The 16-year-old was detained by officials on May 13 near Hidalgo, Texas, after entering the country unlawfully, according to CBP.

He was found 'unresponsive' while being held at the Weslaco Border Patrol Station. The cause of death is unknown, according to CBP.

The agency said the unnamed teen was set to be transferred to the custody of the agency tasked with caring for unaccompanied children who cross the border, the Office of Refugee Resettlement within the U.S. Department of Health and Human Services (HHS).

U.S. law requires unaccompanied children who aren't from Canada or Mexico generally be transferred from border or immigration officials to HHS custody within 72 hours.

An email to CBP on Monday afternoon seeking to clarify the reason for the boy's one-week stay in CBP custody was not immediately answered.

The boy is the fifth Guatemalan child to die in U.S. official custody at the border since December and comes as U.S. border officials are detaining an increasing number of unaccompanied minors and families crossing the southern border.

That figure climbed from roughly 4,968 children in October to nearly 8,897 in April; and from 23,116 families to 58,474 families in the same period, leaving officials scrambling to provide adequate services.